1. Field of the Invention
The invention relates to a liquid crystal alignment agent and uses thereof. Particularly, the invention relates to a liquid crystal alignment agent having good UV stability, and a liquid crystal alignment film formed thereby and a liquid crystal display element having the liquid crystal alignment film.
2. Description of the Related Art
As the consumers' rising quality requirements to the liquid crystal display, the liquid crystal display element is developed toward high performance, in which the requirement of the electrical properties such as liquid crystal alignment property, the voltage holding ratio, the ion density, and image sticking and/or display quality is more even in the past. In addition to having good initial characteristics, the liquid crystal element is also required to be used for a long period of time in harsh environments, especially with good voltage holding ratio and ion density as the main consideration. When the voltage holding ratio of the liquid crystal display element decreases, the display quality cannot fully meet the demands of high contrast and high transmitance. When the ion density is increased, then image sticking and other issues easily occur.
Patent Cooperation Treaty Patent Publication No. WO2008/078796 discloses a liquid crystal alignment film having high voltage holding ratio and low ion density and an N-substituted diamine compound for producing a liquid crystal alignment agent. The N-substituted diamine compound has the structure as shown in Formula (1):

wherein, Ra1 represents a C1 to C5 alkylene group; Ra2 and Ra3 is hydrogen or a C1 to C4 alkyl group, and wherein at least one is a C1 to C4 alkyl group. By using the N-substituted diamine compound, the alignment film produced thereby improves the poor quality of the low voltage holding ratio and high ion density of the conventional liquid crystal display element. However, the liquid crystal alignment film produced by the N-substituted diamine compound cannot be used for a long period of time. If the liquid crystal alignment film is subjected to UV irradiation for a period of time, the voltage holding ratio decreased and the ion density increased, and the low contrast and image sticking also occur in the liquid crystal element.
As shown above, in order to meet the requirements of the properties of the modern liquid crystal element, providing a liquid crystal alignment agent having good UV stability is needed in this field. If applying the liquid crystal alignment film produced thereby in a liquid crystal display element, high voltage holding ratio and low ion density can be maintained with a long-time UV irradiation.